Hydrogen (H2) (also referred to herein as “diatomic hydrogen”) is a clean-energy fuel that has received considerable attention as an alternative to petroleum-based hydrocarbon fuels. Research is advancing hydrogen as an energy source for future applications, for example, in fuel cells, internal combustion engines, hydrogen engines, and hydrogenation engines. In addition, research efforts are being invested in the applications of hydrogen as a reducing agent for purifying harmful waste gases, such as, nitrogen oxides (NOx) and sulfur oxides (SOx). As a consequence, various processes are being examined for hydrogen production.
In a typical process of producing hydrogen, a hydrogen-containing molecule, such as, a hydrocarbon, an alcohol, and/or water, is decomposed using a catalytic reforming reaction, a pyrolysis reaction, or an electrolytic reaction; and the resulting hydrogen atoms combine to yield diatomic hydrogen, which is a useable higher-valued fuel. Disadvantageously, pyrolysis reactions are difficult to stabilize thermally and require extreme temperatures of greater than about 1400° C. Electrolytic reactions disadvantageously require high power consumption and offer slow reaction rates. In order to meet a growing demand for hydrogen, catalytic partial oxidation reforming processes are preferably employed to produce hydrogen fuel. Catalytic partial oxidation reforming processes advantageously avoid the problems associated with pyrolysis and electrolytic reactions.
The prior art considers hydrocarbon fuel reforming processes to be divided into three types of reactions illustrated by the following chemical equations (1) to (3). The first process is a combustion reaction, referred to as “complete oxidation,” wherein steam and carbon dioxide are produced by the reaction of fuel and oxygen. Such reactions take place under fuel-lean conditions in an excess of oxygen.CnHm+(n+¼m)O2→½mH2O+nCO2  Equation (1)If oxygen is restricted to a fuel-rich condition with less than a stoichiometric concentration of oxygen relative to fuel, a second type of process generates hydrogen and carbon monoxide through incomplete or partial oxidation of the fuel.CnHm+½nO2→½mH2+nCO  Equation (2)In a third type of process, which typically occurs when oxygen concentration is substantially reduced, if not essentially zero, hydrogen is generated through the reaction of steam and fuel (known as “steam reforming”).CnHm+nH2O→(n+½m)H2+nCO  Equation (3)The reactions defined by Equations (1) and (2) are exothermic; whereas the reaction defined by Equation (3) is endothermic. Equation (2) can proceed in the absence of a catalyst, although the operating temperature of the non-catalytic process is relatively high. With use of a catalyst, the operating temperature can be lowered, and reaction product can advantageously be generated on reaction equilibrium. The catalytic partial oxidation process of this invention pertains primarily to the reforming reaction illustrated in Equation (2), wherein hydrogen is formed on contact of the fuel with the oxidant under fuel-rich conditions in the presence of a partial oxidation catalyst.
Fuels used in partial oxidation reforming processes include, for example, natural gas, ethane, propane, gasoline, light oil (diesel fuel), and alcohols, such as methanol and ethanol. A suitable reactor for catalytic partial oxidation reforming processes comprises a tube-type flow reactor, as disclosed, for example, in U.S. Pat. No. 6,869,456 B2 and U.S. Pat. No. 6,887,436 B1.
During operation of a catalytic partial oxidation process under fuel-rich conditions in a partial oxidation reactor, coke deposits on the catalyst and on the walls, surfaces, fixtures, and conduits within the interior of the reactor. The term “coke” shall refer herein to any solid carbonaceous by-product resulting from destructive distillation, refining, reforming, and/or oxidation of a petroleum-based hydrocarbon or derivative thereof. The location and quantity of coke deposits depend on many process variables, such as, temperature and flow patterns of reactant and product gases through the reactor and associated conduits. Moreover, coke deposits tend to increase as a function of process operating time (time-on-stream). The quantity of coke deposits at any given time-on-stream is difficult to measure and for all practical purposes unknown. Coke is highly detrimental to the operation of the reactor and catalyst; and if coke is not removed, it can quickly reduce the yield of desirable partially-oxidized product, such as hydrogen. Moreover, coke deposits can eventually clog conduits, nozzles, and orifices and restrict the flow of vapors through the reactor causing reactor pressure to increase to an unacceptable and potentially dangerous level.
The prior art discloses various methods for decoking catalysts and reactors. In one method, as illustrated in U.S. Pat. No. 4,701,429 and U.S. Pat. No. 4,849,025, coke is removed by means of a burn-off cycle with an oxygen-containing gas. This method requires the process of interest to be shut-down for a period of hours, during which time the burn-off cycle is implemented. Shut-down is highly undesirable, because it essentially reduces to zero the yield output of desired partially-oxidized product.
Other prior art, as illustrated in U.S. Pat. No. 4,387,043, teaches a related coke removal method involving transporting a coke-deactivated catalyst from a process reactor of interest to a separate regeneration reactor wherein coke is burned-off under oxygen. Following catalyst clean-up, the regenerated catalyst is transported back to the process reactor of interest. Aside from the complexity and problems associated with transporting solid catalysts to and from a regeneration reactor, this method does not remove coke deposits within the process reactor of interest.
Yet another method of coke removal is disclosed in U.S. Pat. Nos. 4,828,651 and 4,959,126, wherein coke is removed by flushing the coked reactor with steam or with pressurized cold water. Disadvantageously, this method also requires shut-down of the process of interest for several hours if not days, with consequential loss in yield of partially-oxidized product. Moreover, this method is not sufficiently efficient at removing all of the coke build-up.
Prior art methods are taught that attempt in-situ coke removal during operation of the process of interest. WO-A1-2006/074552, for example, discloses locating a heating means within a fuel manifold to burn-off coke within a fuel-conveying member of the manifold, so as to provide in-situ decoking of a gas turbine engine. WO-A1-2010/005633 discloses a process for on-stream decoking of a steam cracking furnace comprising multiple tube banks positioned between a hydrocarbon feedstock inlet and a convection section to radiant section crossover, the decoking process involving the steps of terminating the flow of hydrocarbon feed to at least one of the plurality of tubes and supplying a decoking feed of steam to said tube to effect coke removal.
In yet another method, illustrated in U.S. Pat. No. 4,917,787, a method is disclosed for in-situ decoking in a flame-cracking reactor (ACR process). In the ACR process a hydrocarbon fuel is combusted with oxygen to form a combustion product stream that is mixed with superheated steam to produce a heat carrier. The heat carrier is contacted with a converging hydrocarbon feedstock stream, and the combined mixture is fed into a reaction zone wherein cracking takes place. Periodically, the hydrocarbon stream is shut off, while the reactor temperature is maintained between about 1250° C. and about 1600° C., and the reactor is decoked by means of the heat carrier comprising superheated steam.
More particularly, US 2009/0252661 discloses removing carbon build-up in a reforming reactor by periodic lean operation in deep oxidation mode wherein (i) the oxidation period is on the order of milliseconds and is about 25 percent of the duration of the fuel-rich reforming period, or wherein (ii) the oxidation period is on the order of seconds and is about 10 percent of the duration of the fuel-rich reforming period. Neither of these methods provides effective coke removal while maintaining a high and steady yield of partially-oxidized reaction product.
In view of the above, the art could benefit from an improved method of coke removal during operation of a catalytic partial oxidation process for reforming a hydrocarbon fuel to form a partially-oxidized reaction product, preferably, a useable gaseous hydrogen fuel.